System and method for recovery from memory errors in a medical device

ABSTRACT

A system comprising an implantable medical device that comprises a memory circuit, a radiation detector circuit configured to detect a condition correlative to a high-energy radiation level that exceeds a background radiation level, and a controller circuit. The control circuit checks memory locations for errors using a first rate of error checking per time period during a normal operation mode and, in response to the radiation detector circuit indicating a high-energy radiation level, initiates a memory scrubbing mode, wherein the memory scrubbing mode has an increased rate of error checking substantially all memory locations per time period in the memory circuit to check for any errors and correct any such errors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.11/965,590, filed on Dec. 27, 2007, which is a Continuation of U.S.application Ser. No. 10/806,719, filed on Mar. 23, 2004, now issued asU.S. Pat. No. 7,383,087, which are incorporated herein by reference, andthe priority of each of which is claimed herein.

TECHNICAL FIELD

This patent application relates generally to implantable medical devicesand, in particular, but not by way of limitation, to a system and methodfor reducing the susceptibility of the devices to single event upsets.

BACKGROUND

Implantable medical devices (IMDs) are devices designed to be implantedinto a patient. Some examples of these devices include cardiac rhythmmanagement devices such as implantable pacemakers and implantablecardioverter defibrillators (ICDs). The devices are used to treatpatients using electrical therapy and to aid a physician or caregiver inpatient diagnosis through internal monitoring of a patient's condition.The devices may include electrical leads in communication with senseamplifiers to monitor electrical heart activity within a patient, andoften include sensors to monitor other internal patient parameters.Other examples of implantable medical devices include implantableinsulin pumps or devices implanted to administer drugs to a patient.IMDs often include microcontrollers or microprocessors along with memoryto store program instructions and data. Corruption of data in the memorycan lead to erroneous operation of an IMD.

SUMMARY

This document discusses, among other things, systems and methods fordetecting and correcting memory errors in implantable medical deviceswhen the devices are exposed to high-energy radiation environments. Inone system example, the system comprises an implantable medical devicethat includes at least one electrical input to receive sensed electricalactivity of a heart of a patient, a memory, and a controller circuit.The controller circuit is coupled to the electrical input and memory andis operable to enter a memory scrubbing mode that increases a rate ofdetecting and correcting single bit errors in the memory when thecontroller circuit determines the implantable device is in a high-energyradiation environment.

Another example includes a method that comprises determining that animplantable medical device is in a high-energy radiation environment,enabling a memory scrubbing mode in response to the implantable medicaldevice entering the high-energy radiation environment, and increasing arate of detecting and correcting memory errors in the device upon theenabling of the scrubbing mode. Other examples and advantages are alsodiscussed in the following detailed description and represented in thedrawings that form a part thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings like numerals refer to substantially similar componentsthroughout the several views. Reference numerals with different lettersuffixes represent different instances of substantially similarcomponents.

FIG. 1 illustrates an embodiment of a system that uses an implantablemedical device.

FIG. 2 illustrates an implantable medical device coupled by one or moreleads to heart.

FIG. 3 shows an exemplary embodiment of an implantable medical devicethat is coupled to one or more leads.

FIGS. 4A and 4B are block diagrams illustrating a method 400 fordetecting and correcting memory errors in an implantable medical device.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and specific embodimentsin which the invention may be practiced are shown by way ofillustration. It is to be understood that other embodiments may be usedand structural changes may be made without departing from the scope ofthe present invention.

The present application discusses, among other things, systems andmethods for detecting and correcting memory errors in implantablemedical devices (IMDs) when the devices are exposed to high-energyradiation environments.

Cancer is a common co-morbidity among patients who use IMDs. Thesepatients may be exposed to high-energy radiation cancer therapy. Asmedical technology continues to improve, patients with IMDs will livelonger and the likelihood that they could also become cancer patientsincreases. Exposure to radiation therapy increases the likelihood thatthe implanted devices will experience memory failures. These memoryfailures may be in the form of single event upsets that corrupt randomaccess memory (RAM). The memory failures can occur in memory locationscontaining operating parameters of the IMD or in memory locationscontaining program instructions. These failures may require explantingan IMD from the cancer patient. Manufacturers of IMDs typically labelthe devices to warn a physician or technician to shield the deviceduring the radiation therapy. However, device failure from memorycorruption may still occur from exposing the IMD to backscatteredparticles during the radiation therapy.

FIG. 1 illustrates an embodiment of a system 100 that uses animplantable medical device (IMD) 105. The system 100 shown is oneembodiment of portions of a system 100 used to treat a cardiacarrhythmia. A pulse generator (PG) or other IMD 105 is coupled by acardiac lead 110, or additional leads, to a heart 115 of a patient 120.

Examples of IMD 105 include, without limitation, a pacer, adefibrillator, a cardiac resynchronization therapy (CRT) device, or acombination of such devices. This document relates to any IMD that cantransmit data while implanted in a human or animal patient 120 and isnot limited only to devices that deliver electrical therapy throughleads. This also includes IMDs that are purely diagnostic in nature suchas devices implanted to communicate diagnostic data to an externaldevice. This further relates to IMDs that deliver a drug therapy to apatient.

Cardiac lead 110 includes a proximal end 135 that is coupled to IMD 105and a distal end 140, coupled to one or more portions of a heart 115.System 100 also includes an IMD programmer or other external device 125that provides wireless communication with the IMD 105, such as by usinga telemetry device 130. One embodiment of wireless communication bytelemetry is communication using radio frequency (RF) signals. Anotherembodiment is communication using infrared light signals. Yet anotherembodiment is communication using coils electrically coupled with mutualinductance. In one example of an external device 125, the externaldevice 125 is able to communicate one or more operational parameters tothe implantable device 105 in order to program the IMD 105. If the IMDpatient 120 is also a cancer patient, the patient 120 as well as the IMD105 may be exposed to high-energy radiation therapy 160 as part ofcancer treatment.

FIG. 2 illustrates an IMD 105 coupled by one or more leads 110A-B toheart 115. Heart 115 includes a right atrium 200A, a left atrium 200B, aright ventricle 205A, a left ventricle 205B, and a coronary sinus 220extending from right atrium 200A. In this embodiment, atrial lead 110Aincludes electrodes (electrical contacts, such as ring electrode 225 andtip electrode 230) disposed in, around, or near an atrium 200 of heart115 for sensing signals and/or delivering pacing therapy to the atrium200. Lead 110A optionally also includes additional electrodes, such asfor delivering atrial and/or ventricular cardioversion/defibrillationand/or pacing or resynchronization therapy to heart 115.

Ventricular lead 110B includes one or more electrodes, such as tipelectrode 235 and ring electrode 240, for delivering sensing signalsand/or delivering pacing therapy. Lead 110B optionally also includesadditional electrodes, such as for delivering atrial and/or ventricularcardioversion/defibrillation and/or pacing therapy to heart 115. IMD 105includes components that are enclosed in a hermetically-sealed housingor “can” 250. Additional electrodes may be located on the can 250, or onan insulating header 255, or on other portions of IMD 105, for providingunipolar pacing and/or defibrillation energy in conjunction with theelectrodes disposed on or around heart 115. Other forms of electrodesinclude meshes and patches which may be applied to portions of heart 115or which may be implanted in other areas of the body to help “steer”electrical currents produced by IMD 105. In one embodiment, one ofatrial lead 110A or ventricular lead 110B is omitted, i.e., a “singlechamber” device is provided, rather than the dual chamber deviceillustrated in FIG. 2. In another embodiment, additional leads areprovided for coupling the IMD 105 to other heart chambers and/or otherlocations in the same heart chamber as one or more of leads 110A-B. Thepresent methods and systems will work in a variety of configurations andwith a variety of electrical contacts or “electrodes.”

FIG. 3 shows an exemplary embodiment of an IMD 300 that is coupled toone or more leads, such as bipolar leads 360, 365. The bipolar leads360, 365 include tip electrodes 305A, 305B and ring electrodes 310A,310B. The IMD 300 includes a therapy circuit 315 to deliver electricaltherapy to heart 115 through the leads 360, 365 and electrodes and asensing circuit 320 to sense electrical signals on the leads 360, 365and electrodes. To sense the electrical signals, the sensing circuit 320includes sense amplifier circuits coupled to the switch network 325. Theswitch network 325 is also operable to couple the sensing circuit 320 toany combination of one or more electrodes. The switching network 325further couples the therapy circuit 315 to the electrodes.

Typically, the IMD 300 delivers therapy through the leads or sensesvoltages on the leads, such as between the tip electrodes 305A, 305B andring electrodes 310A, 310B. The IMD 300 includes a controller circuit330 that is operable to connect the therapy circuit 315 and/or sensingcircuit 320 to the electrodes through the switch network 325. In oneembodiment, the controller circuit 330 is operable through logiccircuits implementing a state-machine in hardware. In anotherembodiment, controller circuit 330 includes a processor executinginstructions contained in firmware. In yet another embodiment, thecontroller circuit 330 includes a processor executing softwareinstructions. In yet another embodiment, the controller circuit 330includes any combination of hardware, software and/or firmware.

The IMD 300 also includes a memory circuit 340 coupled to the controllercircuit 330. In one embodiment, the memory circuit 340 includes datastorage for storing operating parameters of the IMD 300. In anotherembodiment, the memory 340 includes firmware or volatile memory such asrandom access memory (RAM) containing instructions executable by thecontroller circuit 330. In another embodiment, the memory circuit 340includes hardware registers accessible by the controller circuit 330. Ifthe IMD patient is also a cancer patient, exposure to high-energyradiation therapy increases the likelihood that the IMD 300 willexperience memory failures due to single event upsets (SEUs). An SEU isa problem that occurs when high density electronics are subjected toradiation fields. For example, an alpha particle impinging on asemiconductor substrate of the memory 340 may generate electron-holepairs that corrupt data stored as a charge on a capacitive element. Thememory location corrupted may contain an operating parameter or anexecutable instruction. This could lead to device failure, possiblyrequiring explanting the IMD 300 from a patient.

To mitigate memory failures in the IMD 300, the controller circuit 330is operable to enter a “memory scrubbing” mode where the controllercircuit 330 detects and corrects memory errors. In normal operation, thecontroller circuit 330 checks for memory errors in the memory locationsit accesses. When in memory scrubbing mode, the controller circuit 330systematically checks locations in memory 340 for memory errors; eventhose memory locations not accessed in normal operation. Systematicallychecking refers to any systematic or uniform method of ensuring that allmemory locations are checked, such as, for example, by incrementing anaddress. In one embodiment, the controller circuit 330 alsosystematically checks memory 340 for errors once-per-day when not in thememory scrubbing mode. This is done to correct rare errors that mayoccur due to normal background radiation levels. However, when thecontroller circuit 330 enters the memory scrubbing mode, the controllercircuit 330 increases a rate of checking for errors. In one embodiment,the controller circuit 330 checks all memory at a rate ofonce-per-second while in memory scrubbing mode. In another embodiment,the controller circuit 330 periodically checks portions of memory.

To detect memory errors in scrubbing mode, the controller circuit 330reads memory 340 and checks the parity of the memory contents. In oneembodiment, the controller circuit 330 reads the memory locations andcompares the parity of the contents to stored parity bits. In oneexample of this embodiment, parity bits are stored for each word ofmemory. In another example, the parity bits are calculated and storedfor an array or matrix of memory words. In another embodiment, thecontroller circuit 330 uses the results of the parity check to detectand correct single bit errors (SBEs). This detection and correction usesa Modified Hamming Code, for example. In another embodiment, thecontroller circuit 330 also detects multiple bit errors (MBEs) and flagsthe occurrence of an MBE. In another embodiment, the controller detectsand corrects MBEs. This detection and correction uses Reed-Solomoncodes, for example.

The amount of additional circuitry required for an IMD to detect andcorrect errors varies with the extent of detecting and checking done inmemory scrubbing mode. In general, detecting and correcting SBEs is lessexpensive in terms of size, complexity and power than detecting andcorrecting MBEs. Because IMDs are usually battery-powered and designedto be of small size, in one example, the IMD only detects and correctsSBEs.

A memory read operation is used to detect an error and a memory writeoperation is used to correct an error. Much of the memory 340 of an IMD300 may be write protected to prevent erroneous code executed by thecontroller circuit 330 or external device 125 from corrupting certaincritical operating parameters in the IMD 300. Such critical operatingparameters are stored in the write-protected region of the memory 340.However, in memory scrubbing mode the controller circuit 330 has readand write access to even the most critical locations of memory tocorrect errors. Error detecting and error correcting operations impactthe longevity of a battery-powered IMD 300. Occasionally executing thisfunction balances power consumption against the likelihood of errorsaccumulating in memory 340. When the IMD 300 is in a high-energyradiation environment, however, the likelihood of errors greatlyincreases and the need for memory scrubbing increases.

In one embodiment, once the controller circuit 330 enters the memoryscrubbing mode, the memory scrubbing is executed continuously incontroller circuit 330 and the memory scrubbing function is given higherpriority than other IMD 300 functions, including those that providetherapy. This embodiment may be useful if the therapy functions arecontained in RAM. In another embodiment, therapy functions are givenhigher priority in the controller circuit 330 than the memory scrubbingfunction. In an example of this embodiment, instructions contained infirmware that provide therapy are given the highest priority forexecution in the controller circuit 330, and instructions to execute thememory scrubbing function use the remaining capacity of controllercircuit 330. In yet another embodiment, a percentage of the capacity ofthe controller circuit 330 is allocated for memory scrubbing. Thispercentage of executing capacity is determined by designers of IMDs tobalance the likelihood of errors against the amount of battery-power,and thus battery life, consumed by executing the memory scrubbingfunction. In yet another embodiment, this percentage of controllercircuit 330 capacity allocated for memory scrubbing is programmed intothe IMD using the external device.

In yet another embodiment, the percentage of capacity used for memoryscrubbing is gradually reduced over time when the controller circuit 330determines that the IMD 300 is no longer in a high-energy radiationenvironment. In this embodiment, the percentage is gradually decreaseduntil the percentage capacity reaches the capacity used for errordetection for normal background radiation energy levels.

To initiate memory scrubbing mode, the controller circuit 330 determinesthat the IMD 300 is in a high-energy radiation environment. In oneembodiment, the controller circuit 330 determines that the IMD 300 is ina high-energy radiation environment by monitoring the number of errorsencountered while accessing memory locations in normal operation mode.The controller circuit 330 compares the rate of memory errors to apredetermined threshold rate and enters memory scrubbing mode when thedetected rate exceeds the threshold rate. The threshold rate isdetermined from characteristics of the device that include, among otherthings, the type of memory in the device and the shielding of thedevice. In an example of this embodiment, the threshold rate ispre-programmed into the IMD 300.

In another embodiment of determining that the IMD 300 is in ahigh-energy radiation environment, the controller circuit 330 is coupledto a radiation detector sensor 345 and monitors the output of the sensor345. In an example of this embodiment, the sensor 345 outputs anelectrical signal to the controller circuit 330 when a radiation levelexceeds a threshold level. In another example, the sensor 345 isimplemented as a radiation-detecting memory cell, or cells, designed tobe more susceptible to radiation than other cells in memory 340. Thecontroller circuit 330 then monitors such radiation-detecting memorycell or cells for an increase in a rate of memory errors in those cellsto determine that the IMD 300 is in a high-energy radiation environment.In one example, the controller circuit 300 determines the IMD 300 is ina high-energy radiation environment by detecting errors in theradiation-detecting memory cells at a rate of one error per minute. Inanother example, the controller circuit 300 determines the IMD 300 is ina high-energy radiation environment when a predetermined number of thecells contain errors. Such radiation-detecting memory cells may bescattered or distributed throughout various physical locations in memory340. Another example is to scatter these radiation-detecting cells asclusters of the cells throughout various physical locations in memory340.

In another embodiment, the controller circuit 300 activates theradiation detector sensor 345 only periodically, effectively placing thesensor in an “arm” mode. This embodiment is useful in a battery-powereddevice to conserve battery energy used to activate the sensor 345. Inone example, the controller circuit 300 is programmable to arm thesensor 345 for the duration of high-energy radiation therapy. In anotherexample, the controller circuit 300 is programmable to arm the sensor345 for a duration sufficient to cover multiple therapies such asdurations of days or weeks.

To exit memory scrubbing mode, the controller circuit 330 determinesthat the IMD 300 is no longer in a high-energy radiation environment. Inone embodiment, the controller circuit 330 detects that the IMD 300 isno longer in a high-energy radiation environment when the rate of memoryerrors falls below a predetermined threshold rate. In an example of thisembodiment, the error rate is determined by monitoring a count of memoryerrors detected during systematic memory reads of the memory 340. Thiscount of memory errors may be determined from only a portion of thememory being systematically read while in scrubbing mode. The exitthreshold rate may be different from the initiation threshold rate. Forexample, the initiation rate may be one error every thirty seconds,while the exit rate may be one error every minute. This permitshysteresis in the process of initiating and exiting memory scrubbingmode.

In another embodiment, the controller circuit 330 is coupled to theradiation detector sensor 345 and the controller circuit 330 exitsscrubbing mode when the sensor 345 indicates a radiation level dropsbelow a threshold level. In yet another embodiment, the radiation sensor345 includes a memory cell, or cells, designed to be more susceptible toradiation than other radiation cells in memory and the controllercircuit 330 exits scrubbing mode when the controller circuit 330 detectsthat a rate of errors in the memory cell or cells has dropped below apredetermined threshold rate. Again, the predetermined exit error ratemay be different from the initiation error rate to provide hysteresis tothe transitioning process.

In yet another embodiment of exiting memory scrubbing mode, thecontroller circuit 330 is coupled to a timer circuit 350 and thecontroller circuit 330 is operable to exit the memory scrubbing modeafter being in the memory scrubbing mode for a predetermined timeduration. This time duration is set to a value longer than the timeneeded to deliver the high-energy radiation therapy. Examples of such atime duration include durations of minutes or hours.

In yet another embodiment of exiting memory scrubbing mode, once thecontroller circuit 330 determines that the device is no longer in ahigh-energy radiation environment, the controller circuit 330 makes asubstantially immediate return to the normal radiation-level environmentrate of detecting and correcting errors. In another embodiment, thecontroller circuit 330 gradually reduces the rate of memory scrubbinguntil the controller circuit 330 returns to the normal radiation-levelenvironment rate.

The controller circuit 330 is coupled to a telemetry circuit 355. Thetelemetry circuit 355 communicates with an external device 125 inFIG. 1. The external device is able to communicate one or moreoperational parameters to the IMD 300. The parameters are then writteninto memory 340 to program the IMD 300. The memory 340 includes RAMand/or hardware registers. One example of setting a predetermined timeduration to be in memory scrubbing mode is to program the time durationinto the IMD 300 using the external device 125. The controller circuit330 then uses the timer circuit 350 to determine when the duration ismet or exceeded to exit the memory scrubbing mode.

In another embodiment of determining that the IMD 300 is in ahigh-energy radiation environment, the external device 125 communicateswith the telemetry circuit 355 to enable a high-energy radiation memoryscrubbing mode in the implantable medical device IMD 300. For example, acare giver would use the external device 125 to enable memory scrubbingmode in the IMD 300 before beginning high-energy radiation therapy. Toexit the memory scrubbing mode, the external device 125 is used todisable the scrubbing mode in the IMD 300.

If the IMD patient is also a cancer patient, another embodiment of theexternal device 125 of FIG. 1 includes an RF transmitter 155 associatedwith a radiation therapy device. Such an RF transmitter 155 transmits anRF indication in the treatment room that causes IMDs or other devices totransition to a memory scrubbing mode. In one embodiment, the RFtransmitter 155 communicates with the telemetry circuit 355 in FIG. 3before radiation therapy to enable the memory scrubbing mode. In anotherembodiment, the RF transmitter 155 transmits continuously during theradiation therapy, and the IMDs or other devices in the treatment roomremain in memory scrubbing mode while the RF transmitter istransmitting. In another embodiment of the external device 125, theexternal device 125 is operable to communicate with a global computernetwork.

FIG. 4A is a block diagram illustrating a method 400 of detecting andcorrecting memory errors in an IMD 300. At 410, an IMD 300 determines itis in a high-energy radiation environment. For example, a high-energyradiation environment is determined when the IMD 300 detects a number oferrors that exceeds a threshold while the IMD 300 is in normaloperation. In another example, the IMD 300 includes a radiation-detectorsensor 345 and the sensor 345 provides an indication that the IMD 300 isin a high-energy radiation environment. In yet another example, the IMD300 detects a number or rate of memory errors that occur in aradiation-detecting memory cell or cells. In yet another example, theIMD 300 determines it is in a high-energy radiation environment bycommunication with an external device 125. At 420, a memory scrubbingmode is enabled in response to the implantable medical device enteringthe high-energy radiation environment. At 430, a rate of detecting andcorrecting memory errors is increased in the IMD 300 upon the enablingof the scrubbing mode.

FIG. 4B shows a further embodiment of the method 400. At 440, the IMD300 determines it is no longer in the high-energy radiation environment.For example, the IMD determines it is no longer in a high-energyradiation environment when the IMD 300 detects a number of errors thatis below a threshold number while the device is in a memory scrubbingmode. In another example, the IMD 300 includes a radiation-detectorsensor 345 and the sensor 345 provides an indication that the IMD 300 isno longer in a high-energy radiation environment. In yet anotherexample, the IMD 300 detects that a number or rate of memory errors thatoccur in a radiation-detecting memory cell or cells is below a thresholdnumber or rate. In yet another example, the IMD 300 determines it is nolonger in a high-energy radiation environment by communication with anexternal device 125. In yet another example, the IMD 300 determines itis no longer in a high-energy radiation environment by expiration of atimed duration. At 450, the memory scrubbing mode is disabled. At 460,the IMD 300 returns to a lower rate of detecting and correcting memoryerrors in the device.

The accompanying drawings that form a part hereof, show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced. The embodiments illustrated aredescribed in sufficient detail to enable those skilled in the art topractice the teachings disclosed herein. Other embodiments may beutilized and derived therefrom, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations, or variations, or combinations of variousembodiments. Combinations of the above embodiments, and otherembodiments not specifically described herein, will be apparent to thoseof skill in the art upon reviewing the above description.

The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of the technical disclosure. It is submitted withthe understanding that it will not be used to interpret or limit thescope or meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a single embodiment for the purpose of streamlining the disclosure.This method of disclosure is not to be interpreted as reflecting anintention that the claimed embodiments require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive subject matter lies in less than all features of asingle disclosed embodiment. Thus the following claims are herebyincorporated into the Detailed Description, with each claim standing onits own as a separate embodiment.

1. A system comprising: an implantable medical device, the implantablemedical device comprising: a memory circuit; a radiation detectorcircuit configured to detect a condition correlative to a high-energyradiation level that exceeds a background radiation level; and acontroller circuit, communicatively coupled to the memory circuit andthe radiation detector circuit, wherein the control circuit isconfigured to: check memory locations for errors using a first rate oferror checking per time period during a normal operation mode; and, inresponse to the radiation detector circuit indicating a high-energyradiation level, initiate a memory scrubbing mode, wherein the memoryscrubbing mode has an increased rate of error checking substantially allmemory locations per time period in the memory circuit to check for anyerrors and correct any such errors.
 2. The system of claim 1, whereinthe radiation detector circuit includes at least one memory cell of thememory circuit that is susceptible to lower levels of radiation energythan other memory cells of the memory circuit, and wherein the radiationdetector circuit indicates a high radiation environment when having arate of memory errors that exceeds a specified threshold memory errorrate.
 3. The system of claim 2, wherein the radiation detection circuitincludes a plurality of memory cells susceptible to lower levels ofradiation energy than other memory cells of the memory circuit, andwherein the controller circuit is configured to detect, at a firstchecking rate, a rate of occurrence of errors in information stored inthe memory circuit, and to compare the rate of occurrence of errors to apredetermined threshold, and to enter the memory scrubbing mode toincrease the checking rate from the first checking rate value per timeperiod to a second checking rate value per time period in response tothe rate of occurrence of the errors exceeding the predeterminedthreshold.
 4. The system of claim 1, wherein in response to theradiation detector circuit indicating a high-energy radiation level, thecontroller circuit checks the memory circuit for any errors and correctsany such errors, the checking and correcting performed more frequentlywhen the radiation detector circuit indicates a high-energy radiationlevel is present than when the radiation detector circuit indicates thata high-energy radiation level is not present.
 5. The system of claim 1,wherein in response to the radiation detector circuit no longerindicating a high-energy radiation level, the controller circuit checksthe memory circuit for any errors and corrects any such errors lessfrequently than when the radiation detector circuit indicates that ahigh-energy radiation level is present.
 6. The system of claim 5,wherein the radiation detector circuit includes at least one memory cellof the memory circuit that is susceptible to lower levels of radiationenergy than other memory cells of the memory circuit, and wherein theradiation detector circuit no longer indicates a high radiationenvironment when having a rate of memory errors less than a specifiedexit threshold memory error rate.
 7. The system of claim 6, wherein theexit threshold memory error rate is equal to a memory scrubbing modeinitiating threshold memory error rate.
 8. The system of claim 1,wherein the radiation detector circuit includes the controller circuitand the memory circuit operating together to detect a rate of occurrenceof errors in memory, wherein the controller is configured to compare adetected rate of occurrence of errors in memory in the memory circuit toa specified threshold value and to declare a high-energy radiation levelif the detected rate exceeds the specified threshold value.
 9. Thesystem of claim 1, wherein in response to the radiation detector circuitindicating a high-energy radiation level, the controller circuit checksthe memory circuit for any single bit errors in memory and corrects anysuch single bit errors.
 10. The system of claim 1, wherein theimplantable medical device includes: at least one port to providetherapy to the patient; and a therapy circuit coupled to the at leastone port and the controller circuit, the therapy circuit operable todeliver therapy to the patient, wherein the controller circuit isconfigured to withhold therapy when the implantable medical deviceenters the memory scrubbing mode.
 11. A method of operating animplantable medical device comprising: checking memory locations forerrors using a first rate of error checking per time period during anormal operation mode; detecting a condition correlative to ahigh-energy radiation level that exceeds a background radiation levelusing a radiation detector circuit in the implantable medical device;and initiating a memory scrubbing mode in the device, wherein the memoryscrubbing mode includes an increased rate of error checkingsubstantially all memory locations per time period to check for anyerrors and to correct any such errors.
 12. The method of claim 11,wherein detecting a condition correlative to a high-energy radiationlevel using the radiation detector circuit includes the implantabledevice detecting that one or more memory cells, that are susceptible tolower levels of radiation energy than other memory cells, have a rate ofmemory errors that exceeds a specified threshold memory error rate. 13.The method of claim 11, wherein detecting a condition correlative to ahigh-energy radiation level includes: detecting, at a first checkingrate, a rate of occurrence of errors in information stored in the memorylocations, wherein the memory locations include a plurality of memorycells that are susceptible to lower levels of radiation energy thanother memory cells of the memory, and comparing the rate of occurrenceof errors to a specified threshold memory error rate, and whereininitiating the memory scrubbing mode includes entering the memoryscrubbing mode to increase the checking rate from the first checkingrate value per time period to a second checking rate value per timeperiod in response to the rate of occurrence of the errors exceeding thespecified threshold memory error rate.
 14. The method of claim 11,including checking memory locations at a rate less than the increasedrate of error checking per time period in response to no longerdetecting the high-energy radiation level.
 15. The method of claim 14,wherein no longer detecting the high-energy radiation level includesdetecting, in memory cells that include at least one memory cell of thememory circuit being susceptible to lower levels of radiation energythan other memory cells, a rate of memory errors less than a specifiedexit threshold memory error rate.
 16. The method of claim 15, whereinthe exit threshold memory error rate is equal to a threshold memoryerror rate that initiates the memory scrubbing mode.
 17. The method ofclaim 11, wherein checking all memory locations per time period for anyerrors includes checking for any single bit errors in memory andcorrecting any such single bit errors.
 18. The method of claim 11,including withholding a device-based therapy when the implantablemedical device enters the memory scrubbing mode.
 19. The method of claim11, wherein initiating the memory scrubbing mode includes executinginstructions implementing the memory scrubbing mode at a lower prioritythan instructions related to a device-based therapy.
 20. A systemcomprising: an implantable medical device, the implantable medicaldevice comprising: a memory circuit; means for determining a conditioncorrelative to a high-energy radiation level that exceeds a backgroundradiation level; and a controller circuit, communicatively coupled tothe memory circuit and the radiation detector circuit, wherein thecontrol circuit is configured to: check memory locations for errorsusing a first rate of error checking per time period during a normaloperation mode; and, in response to the radiation detector circuitindicating a high-energy radiation level, initiate a memory scrubbingmode, wherein the memory scrubbing mode has an increased rate of errorchecking substantially all memory locations per time period in thememory circuit to check for any errors and correct any such errors.